The present invention relates to improved prepregs and methods of producing them. In particular, the invention relates to improved printed circuit wiring boards having a low dielectric constant prepared from such prepregs, and to methods for producing such printed circuit wiring boards.
Conventional printed circuit wiring boards are prepared from "prepregs" which, in turn, are typically prepared by pretreating a cloth substrate, e.g., a fiberglass cloth substrate, with a polymer resin, as by dipping in a solution of resin. The resin is selected so as to provide good strength and low dielectric constant. The solution is then dried to remove the solvent and provide a resin-impregnated prepreg or substrate. Advantageously, the glass substrate is treated with a silane compound to promote adhesion between the substrate and the resin. Such laminates are compared in the market place for such factors as dielectric constant, dissipation factor, chemical resistance, peel strength, solder bath resistance (resistance to delamination when immersed in molten solder), warping and punchability. It is particularly desirable that the dielectric constant of such laminates be low. Accordingly, skilled workers increasingly seek to lower the dielectric constant of such laminates and increase the bond strength between laminations.
CA 106:51353g discloses laminated printed circuit boards having good heat resistance and low dielectric constant comprising non-woven glass fabric impregnated with polybutadiene and diallyl phthalate.
Fibrous materials, e.g., cellulosic and fiberglass woven materials, have long been used to reinforce polymer substrates. It is also known that silane coupling agents can be applied directly to glass filaments to improve the flexural strength of laminates prepared from glass cloth impregnated with polymeric resins. Typical strength increases can be as much as 300% for compression molded test samples. Silane coupling agents have also been employed with the minerals which are used as reinforcing fillers in composites to increase strength, hardness, modulus, heat distortion and impact strength. Fiberglass cloth is typically treated with an aqueous coupling agent.
In the preparation of printed circuit wiring boards using prepregs, two or more prepregs, prepared as discussed above, are pressed together to form an insulating layer for a printed circuit wiring board. To provide the conductive layer, one or more layers of a conductive film, e.g., a copper film, are placed on the outside of the prepregs and laminated to the prepregs at the same time the prepregs are laminated to each other. Alternatively, the conductive film can be applied by vapor deposition, electroplating, sputtering, ion plating, spraying and layering. Typical metals employed include copper, nickel, tin, silver, solder, gold, aluminum, platinum, titanium, zinc and chrome, with copper being used most often in printed wiring boards.
A problem associated with such constructions is the difficulty in applying conductive films so that they bond well to the surface of the prepreg. In fact, prior workers have not been able to form a complete bond having excellent bond strength between the metallic layer and the substrate, and subsequently good solder resistance.
Silane compounds have found wide acceptability for improving adhesion between different substrates. Silane coupling agents modify the interface between metal or mineral surfaces and organic resins to improve adhesion between the surface and the resin. The physical properties and water resistance of the reinforced resins are thereby improved. It is believed that silane coupling agents form bonds with metal surfaces through the silane functional group. The hydrolyzed silanes will condense to oligomeric siloxanols and eventually to rigid cross-linked structures. Contact with a polymer matrix should take place while the siloxanols still have some solubility. Bonding to a polymer matrix may take different forms or a combination of forms. Bonding may be covalent where the oligomeric siloxanol is compatible with the liquid matrix resin. The solutions might also form an interpenetrating polymer network as the siloxanols and the resin separately cure with only limited copolymerization.
Prepregs composed of copolymers have been employed in the past. For example, CA 106:51353g discloses printed circuit laminates comprising 1,2-polybutadiene and diallyl phthalate or polymers thereof.
CA 106: 51354h discloses adhesive compositions employing isobutylene-maleic anhydride-styrene terpolymers.
It is well known that not all silanes or mixtures of silanes will bond all metals to all substrates. McGee, 4,315,970, states that "it is generally accepted that specific silanes can be used for adhesion of specific materials to specific substrates. That is, the silane must be matched to the application and it cannot be assumed that all silanes will work in all applications." Therefore, the suitability of a silane bonding agent in improving adhesion of a metal to a substrate is unpredictable and it must be determined by experimentation.
While suitable coupling agents are commercially available for bonding of many common plastics with a variety of metals, the application of silane coupling agents for bonding of polynorbornenes to metals is not previously known. Norbornene type monomers are polymerized by either a ring-opening mechanism or by an addition reaction wherein the cyclic ring structure remains intact. Ring-opening polymerizations are discussed with greater particularity in U.S. 4,136,247 and 4,178,424, assigned to the same assignee as the present invention and incorporated herein by reference for their discussion of such polymerizations. Ring-opening polymerization generally yields unsaturated linear polymers while addition polymerization yields polycycloaliphatics. It is desirable to produce polymers having high molecular weight monomers incorporated therein to provide good temperature resistance, i.e., high heat distortion temperatures and high glass transition temperatures.